With the progress and advance of electronics in the fields of portable personal computers, mobile phones, and personal digital assistances (PDA) in recent years, a secondary battery which can be repeatedly charged and discharged is in wide use as an electric storage device for these electronic devices.
Among the secondary batteries, the so-called rocking chair type lithium ion secondary battery is particularly in wide use as an electric storage device for such electronic devices as mentioned above because of the reasons as follows. The so-called rocking chair type lithium ion secondary battery uses as an active material a lithium-containing transition metal oxide such as lithium manganate or lithium cobaltate for a cathode and a carbon material into which lithium ions can be inserted and from which lithium ions are extracted for an anode so that while it is charged and discharged, the lithium ion concentration in the electrolytic solution does not substantially change. Thus, the rocking chair type lithium ion secondary battery needs electrolytic solution in an amount smaller than the so-called reserve type secondary battery. Such being the case, the rocking chair type lithium ion secondary battery can be downsized more readily than the reserve type secondary battery. Furthermore, the rocking chair type lithium ion secondary battery has a high energy density.
However, the lithium ion secondary battery is an electric storage device which obtains electric energy by electrochemical reactions. Thus, it has a material problem that it has a low power density because the rate of the electrochemical reactions is small. In addition, because the lithium ion secondary battery has a high internal resistance, it can be hardly charged and discharged rapidly.
Besides, as the lithium ion secondary battery contains a cathode active material having a large specific gravity, it leaves room for improvement of capacity density per unit weight, and on the other hand, as the electrode and electrolytic solution deteriorate due to electrochemical reactions when the battery is charged and discharged, it has an insufficient life, or cycle characteristics.
Under these circumstances, a nonaqueous electrolyte secondary battery in which a conductive polymer such as polyaniline having a dopant is used as a cathode active material is already known (see Patent Document 1).
However, in general, a nonaqueous electrolyte secondary battery which comprises a conductive polymer as a cathode active material cannot contribute to downsizing of a secondary battery. The reason is that because the nonaqueous electrolyte secondary battery which comprises a conductive polymer as a cathode active material is an anion-migrating type battery in which the conductive polymer is doped with anions when the battery is charged, and the anions are dedoped from the polymer when the battery is discharged, a rocking chair type secondary battery cannot be constructed when a carbon material is used as an anode active material into which lithium ions are inserted and from which lithium ions are extracted, and hence the battery needs a large amount of electrolytic solution.
In order to solve the problem mentioned above, a cation-migrating type secondary battery in which a cathode is formed of a conductive polymer having polymer anions such as polyvinylsulfonic acid as a dopant so that the ion concentration in the electrolytic solution remains substantially unchanged is proposed (see Patent Document 2). But the battery performance is not yet sufficient.
On the other hand, in recent years, a strategy for solving a problem of air pollution and, even a strategy for solving a problem of global warming, are earnestly studied. As one of the strategies, a hybrid vehicle and an electric vehicle have already reached a stage of practical use, and a lithium ion secondary battery has been put to practical use in part as an electric storage device for such vehicles.
However, although an electric storage device for hybrid or electric vehicles is required to have a high power density in particular when it is rapidly charged through a process of generative brake, or when a vehicle is accelerated, a lithium ion secondary battery has a high energy density, but it has a problem of low power density, as set out hereinbefore.
An electric double layer capacitor thus attracts attention. The electric double layer capacitor is an electric storage device which uses a polarizable electrode usually formed of a conductive and porous carbon material having a large specific surface area such as powder charcoal and fibrous charcoal, and which makes use of physical adsorption characteristics of supporting electrolyte ions in electrolytic solution. Therefore, the electric double layer capacitor has a high power density and is capable of being charged rapidly, and besides it has a very long life. However, on the other hand, as it has an energy density much smaller than a lithium ion secondary battery, it is problematic if the electric double layer capacitor can be put to practical use as an electric storage device for hybrid or electric vehicles.
For example, the electric double layer capacitor has a cycle life about 10-100 times longer, and a power density about 5 times larger, than a lithium ion secondary battery; however, the electric double layer capacitor has a weight energy density about 1/10-½ as much as a lithium ion secondary battery, and a volume energy density about 1/50- 1/20 as much as a lithium ion secondary battery (see Patent Document 3).